2-bit Paper Processor Teaches How They Work

Take a few minutes out of your day, grab your scissors, and learn how a simple processor works. [Saito Yutaka] put together an exercise to teach processor operations with paper. After downloading the PDF you can cut out the Address and Data pointer as well as two-bit data tokens for each. The processor has three instruction sets; Increment register by one, Jump if not over flow, and Halt wait for reset.

Once you’ve got your cutouts you can follow along as the program is executed. The INC operation is run, with the JNO used to loop the program. Once the register has reached an overflow the overflow counter halts the program.

One word of warning, we think there’s a typo in one of the captions.  Once the program starts running and gets to address 01(2) the caption still reads 00(2) for both address and data. As long as you compare the values in the picture along the way you should have no problem getting through execution. which has now been fixed.

17 thoughts on “2-bit Paper Processor Teaches How They Work

  1. this is cool and all but why does the example program work in the pictures but his instructions are always 00(2)

    >>Fetch the instruction . Read data on address 00(2) .
    Data is 00(2). This means increment register by one

    >>Fetch the instruction . Read data on address 00(2) .
    Data is 00(2). This means jump if not over flow .

    lol wat

  2. Thank you Hackaday. I’m a TA in an Assembly class and I’ve been needing a visual aid to teach to high school students.

    I may have to do some expanding to cover all that I need to though.

  3. There are a few errors in the writeup and this page’s description, but overall a very neat way to demonstrate how a processor works at the lowest level.

    Well, I suppose you could get into how the transistors and cache store the data on the silicon, but that’s not so much computing at that point.

  4. explain this:
    Fetch the instruction . Read data on address 01(2) .
    Data is 01(2). This means jump if not over flow . Status register is 0 . So CPU should jump to somewhere .
    But CPU doesn’t know which address to jump .


    Increment program counter .


    Read data on address 10(2) . The data value is 00(2) .
    CPU know a address to jump .

    how does it now know to jump? BUT why does it(jump) when the instruction(00) is to increment the register by one ???

    maybe edit to say “01 = jump if not status else jump to next read data address” or something in programmer speak

    1. Well, you probably took it from the bad end. The 01b is a 2-word (where word means a group of 2 bits for now) instruction. In case the condition is met, the processor jumps to the address specified in the operand in second word of the 01xxb instruction. In case the condition is NOT met, then it jumps to the next instruction. Therefore it jumps 2 words further, effectively jumping to the address 11b. Here it reads the 10b instruction, thereby halting the system.

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